Arctic Sea Ice Measurements

Arctic Sea ice extent is virtually impossible to accurately measure from the Earth's surface. The edges of the ice are ever changing and the sheer size of the ice mass (averaging two and half times the size of Canada) makes it difficult to measure directly on short time scales. To overcome the shortcomings of in situ observations, polar orbiting satellites began collecting data over the Arctic (as well as the Antarctic) in the 1970s. Scientists use radiometry data and visible imagery collected from the satellites to determine the sea ice extent. Each technique has its advantages and disadvantages, and more information can be found through the National Snow and Ice Data Center. Today a suite of NASA, NOAA, and Department of Defense satellites provide the data which is needed to accurately monitor sea ice extent on a daily, monthly, and annual basis.

The transition from ice–covered to ice–free ocean can occur over a large distance. When measuring the Arctic Sea ice extent from satellites, a threshold of minimum ice concentration is defined to mark where the ice sheet ends. NOAA uses a threshold of 15 percent ice concentration over an areal extent, because it provides the most consistent agreement between satellite and ground observations. At this low ice concentration, ocean waters are generally navigable by ships, one of the earliest motivations for better understanding changes in Arctic ice.